1. Field of the Invention
The present disclosure relates to staged fuel systems, and more particularly to staged fuel systems such as used in gas turbine engines.
2. Description of Related Art
A variety of devices and methods are known in the art for injecting fuel into gas turbine engines. Of such devices, many are directed to injecting fuel into combustors of gas turbine engines under high temperature conditions while reducing or preventing carbon formation or coking within fuel passages.
It is well known that fuels typically used in gas turbine engines are susceptible to breaking down into their constituent components when exposed to high temperatures. For example, if wetted wall temperatures in fuel lines utilizing Jet A fuel are allowed to reach about 350° F., the fuel will begin to break down, which can lead to carbon formation within the fuel lines. This phenomenon is known as coking. Since coking can build up and constrict and eventually block fuel passages, and since coke may flake off resulting in a large foreign object which can then clog or plug sensitive portions of the fuel injector, considerable efforts have been made to protect fuel circuits in fuel injectors and nozzles from high temperatures within gas turbine engines.
Staged fuel injectors for gas turbine engines typically include a pilot fuel atomizer for use during engine ignition and low power operation and at least one main fuel atomizer for use during higher power operation in concert with the pilot fuel atomizer. One difficulty associated with operating a staged fuel injector is that when the pilot fuel circuit is operating alone during low power operation, stagnant fuel located within the main fuel circuit can be susceptible to coking due to the temperatures associated with the operating environment. This can degrade engine performance over time if the main fuel circuitry becomes constricted or blocked from coking, as described above. To address this, maintenance can be performed on the injectors.
High performance gas turbines often utilize circumferential or radial staging of fuel injectors to help limit emissions and maintain stability at part power conditions. Staging injectors circumferentially is sometimes referred to as selective fuel injection, which can be accomplished through injector to injector fuel control, staging within a multi-circuit injector, or some combination thereof. This technique can be used with piloted or non-piloted fuel injectors. As an example, at low throttle conditions only half or less of the injectors of a combustor may be used to actually issue fuel. Fuel within the remaining nozzles not issuing fuel to the combustor is susceptible to coking, just as stagnant circuits in staged fuel injectors described above. Coking can occur during selective fuel injection regardless of what type of fuel injectors are used.
In the past, attempts were made to extend injector life by passively insulating, active cooling, or otherwise protecting the fuel circuitry of fuel injectors from carbon formation during low power engine operation. Passive insulation often involves incorporation of heat shielding into injector design. Purging stagnant fuel from unused fuel circuits is another possible way of extending injector life, but this requires a pressure source, typically compressor discharge air ingested by the injector, to force stagnant fuel from the circuit. For staged fuel injectors, efforts have also been made to actively cool stagnant fuel using fuel flow from a pilot fuel circuit. This usually involves use of a second source such as a cool fuel line or cooled air flow to remove heat from stagnant fuel circuits. One such effort is disclosed in U.S. Pat. No. 5,570,580 to Mains, which provides a fuel injector having two dual orifice injector tips, each with a primary and secondary pressure atomizer. In this injector, fuel streams to the primary and secondary sprays of the pilot and main nozzle tips are arranged to transfer heat between the pilot primary fuel stream and each of the main secondary fuel stream and the pilot secondary fuel stream. However, in these active cooling techniques, the heat capacity of the cooling media and its proximity to the stagnant fuel circuit can limit the effectiveness of actively cooling the stagnant circuit.
U.S. Patent Application Publication No. 2007/0163263 to Thomson, incorporated by reference herein in its entirety, describes a staged airblast fuel injector designed for active cooling. Fuel passing through the pilot fuel circuit is routed through the injector to cool stagnant fuel within the main fuel circuit during low power engine operation. This configuration can reduce or prevent carbon formation or coking in the main fuel circuit of the injector.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. However, there is still a need for improved systems and methods for thermal management of staged fuel systems. The present disclosure provides a solution for these problems.